Our principal objective is the determination of the three-dimensional structures of immunoglobulins from patients with multiple myeloma and/or amyloidosis. We are currently investigating a serum IgG1 protein from the same patient (Mcg) who provided the urinary Bence-Jones dimer for which we previously solved the structure and constructed an atomic model. By selecting the IgG1 protein from Mcg, we can make extensive use of the known structure of the Bence-Jones protein, which has the same amino acid sequence as the light chains in the parent IgG1 molecule. The Bence-Jones dimer is sufficiently similar to antigen-binding fragments (Fab) in size, shape, and the ability to bind hapten-like molecules that it can be considered a mode for a primitive antibody. A continuing comparison of the bindng properties of the Mcg Bence-Jones dimer and the IgG1 protein has provided important insights into the structural basis of antibody specificity. The Fab and Fc regions are clearly defined in a 6.5-Angstrom units electron density map of the IgG1 protein. In contrast, a crystallographic comparison of the Zie IgG2 cryoglobulin and its F(ab')2 fragment indicates that the Fc region does not contribute significantly to the diffraction pattern of the IgG2 immunoglobulin.